GENERATOR PROTECTIONS

PROTECTIONS OF GENERATOR
Prepared by: Dhaval D. Julasana (ELECTRICAL-GET)
Thankful to Electrical department
Date: 23/10/2018
NIRMA LTD. BHAVNAGAR

Introduction
Type of power generation
Construction and working of generation
Excitation system
Automatic voltage regulator
Type of generators
Difference between salient pole and cylindrical pole generator
Why protection required?
Generator Protections
Generator connection circuit
Generator with NGT
Various faults
Abnormal operating conditions
Excitation of generator field
Stator fault
Differential protection
Percentage bias differential protection
Rotor fault
Protection against rotor fault
Over speeding
Loss of excitation
Contents

• Protection against prime mover failure
• How to provide direction into the relay
• Failure of excitation
• Torque develop strategy
• Why protection is required?
• Earth fault
• Types of earthing
• High impedance earthing
• Compensated
• Hybrid impedance ground earthing
• Rotor earth fault
• Why do we care of nearer earth fault
• Pole sleeping
• Stator ground fault

Introduction
 Power system is Biggest men made system in the world
 Among this biggest Power system is in india
 Installed capacity of the Power system is approx. 344GW (August
2018) ,329.20 Gw (april 30th 2017 , At year of 1941 generation was
1362MW
 What is the meaning of grid?
Interconnection of the power system components through ac line
 How many grids in India?
Earlier ES= Eastern grid
NR= North Region
SR= South region
NER= North eastern region

• At first synchronization done with ES-NER
• Then NR-ES-NER
• WR-NR-NER-ES
• Above combined region called central region
• At last only two region remaining which is CE & SR
• Both have separate frequency
• 31st December 2013 around 20:00 Hrs Southern grid successfully synchronized
with central grid

Type of power generation
Majority three type of generation does in india.
• Thermal
• Nuclear
• Water
• World’s largest solar plant currently building up in to Madhya Pradesh
Name is “RIVA”, It’s total capacity 750MW.

76%
10%
4%
2%1%3% 4%0%
GENERATION
COAL
LARGE HYDRO
WIND POWER
SOLAR POWER
BIO MASS
NUCLEAR
GAS
DIESEL
Generation data of (2018)
COAL= 986591 Gwh
HYDRO=126123Gwh
Wind power= 52666 Gwh
Wind power = 52666
Solar power= 25871 Gwh
Biomass= 15252Gwh
Nuclear= 38346 Gwh
Gas= 50208Gwh
Diesel= 386Gwh

56.90%
14.50%
9.90%
6.70%
2.60% 2%
7.20%
0.20%
CATEGORY 1
INSTALLED CAPACITY
COAL HYDRO WIND POWER SOLAR POWER BIOMASS NUCLEAR GAS DIESEL
Installed capacity of plants (2018)

Construction and working of generator
• Alternator is one kind of transducer which converts mechanical energy to electrical energy
working on the principle of ELECTROMAGNETIC INDUCTION
• Alternating current will produce by rotating magnetic field so, DC field will hold on rotor
and armature on stator so static dc flux will link on the armature and it will produce the
alternating power.

Excitation system
• Excitation of generator means current will pass through field winding so it will
produce the flux for synchronous generator DC current is needed for that one extra
pilot generator is there so it will provide DC supply to field of generator.
• Pilot generator connected with same shaft. And it’s output is rectified through
solid state rectifier connected with the slip-ring of main generator rotor
• Pilot generator’s output is AC output and it’s need to rectify and convert in to DC
for main exciter and again main exciter’s output need to convert in to DC by diode
bridge circuit and finally this DC output will feed into main machine.

Internal circuit

Automatic voltage regulator

Synchronous Generator
• Synchronous generator converts mechanical power into electrical power source of
mechanical power maybe any thing it may be diesel engine, steam turbine or any similar
device.
• For high- speed machine the prime movers are usually steam turbine employing fossil
or nuclear energy resources
• Lower speed machine are often driven by hydro-turbines that employ water power for
generation.
• Smaller synchronous machine are sometimes used for private generation and at stand by
units, with diesel engines or gas turbines as prime movers.

Types of Generator
• Cylindrical-rotor( non-salient pole)- Machines used in Thermal Power plant
generators. Number of poles will be less. These type of generator are High Speed
generator.
• Salient-Pole machines used in hydro power plantNumber of pole will be High.
These type of generator are low speed generator.

Difference between Salient pole and cylindrical pole
generators
• Salient pole generator
1. It has large diameter and shorter
axis length.
2. As the rotor speed is lower, more
number pole are required to attain
the required frequency frequency is
proportional to number of poles.
3. Flux distribution is relatively poor
than non salient pole rotor, hence
the generator emf waveform is not
as good as cylindrical rotor
4. Salient pole rotors generally need
damper windings to prevent rotor
oscillations during operation.
• Cylindrical pole generator
1. They having smaller diameter but
having longer axial length.
2. Cylindrical rotors are used inhigh
speed electrical machines, usually
1500rpm to 3000rpm.
3. Winding loss as well as noise is less
compare to salient pole rotors.
4. Number of poles is usually 2 or 4.
5. Damper windings are not needed in
Cylindrical pole rotors.
6. Flux distribution is symmetrical so
sinusoidal waveform is there so
better emf waveform.NIRMA LTD. BHAVNAGAR

Why protection?
1. To detect fault because huge current will damage the equipment
2. To prevent injury to personal
3. To prevent damage of equipment
4. To monitor parameter continuously
5. Operate quickly whenever necessary

How does protection system works?

Generator protection
Generator protection is challenging problem because it’s system connections on three
different sides as shown in fig1 on the ne side is prime mover second side it is grid third
side for dc excitation system
It has to run synchronism with the grid because of its connection to the power system. On
yet another side generator protection is very complex compared to protection for other
elements of the power system.
In case of a fault on turbo alternator, it is not enough to open main circuit breaker
connecting it to power grid. Ex. When turbo alternator driven by a steam turbine is tripped,
the following must be done.
• Steam supply to the turbine is stopped or bypassed.
• Firing of the boiler is stopped.
• Coal mills are stopped;
• Field circuit of the alternator is interrupted.
• Alternator kept running at slow speed with the help of barring gear till it cools down
uniformly, so as to avoid uneven expansions.

COMPLEXITIES OF GENERATOR PROTECTION

Generator protection depends on following:
• Type of prime mover and it’s construction
• MW and voltage rating
• Mode of operation
• Method of connection to the power system
• Method of earthing

Electrical circuit of the generators

Continue…
• Please noted that generator is never solidly grounded
• If ground it solidly then at fault like L-G fault time abnormally very high current
will flow from neutral point to rotor it will generate very high vibration in rotor
and mechanically damage it and asymmetric field occurs in it.
• So in practical practice we will ground through NGR or NGT.
• It will limit the fault current
• UAT will provide auxiliary supply to the generators auxiliary equipment, it’s
generally power supply 10% of full load power of generator. In our 82MW power
plant Here 36%

Generator with NGT

Electrical circuit of field exciter of generator

Various faults
Electrical
faults
Three phase
stator winding
Phase fault
Inter turn
fault on the
same phase
Ground faults
Field widing
Short circuit
to ground

Abnormal operating condition
Abnormal
operating
condition
Electrical
Three phase
stator
winding
Unbalance
loading
Field winding
Loss of
excitation
Mechanical
Over
speeding
Loss of prime
mover

Stator faults
• There is possibility to occurs inter turn fault, phase to phase fault and phase to ground
fault.
• For that longitudinal protection.

Differential protection
• Differential protection make for unit protection system, This protection should
operate due to internal fault of unit but not external fault.
• In healthy condition zero current will flow from operating coil.
• But due to construction of the CT no one CT can same as in terms of construction
there is always error in plus or minus terms. So from operating coil always current
will flow I1-I2 value.
• So due to external fault far from transformer some inrush current will flow so, I1-
I2 current’s value will increases upto relay’s peak up value so mal-operation does
occurs.
• To remove this malfunction need to change this characteristics.
• So it is percentage bias differential relay.

PERCENTAGE BIAS DIFFERENTIAL
PROTECTION

Longitudinal differential protection

Difference between transformer and generator
differential protection
• POWER TRANSFORMER
1. Primary and secondary voltages are
in general different.
2. Turn-ratio of the CTs are different
because of ratio of transformation of
the transformer
3. Tap changer may be present
• GENERATOR
1. Same voltage for CTs on two sides
of generator winding
2. Turn-ratio of CTs on the two sides
of the generator winding is same
3. No such device is present

Rotor faults
• The rotor carries the field winding which is kept isolated from the ground.
Neither the positive nor the negative terminal of the dc supply is grounded.
Thus, any ground fault on the rotor field winding does not affect the working
of the alternator.
• However subsequent fault would cause a section of the rotor winding to be
short circuited, giving rise to a secondary flux which oppose the main flux.
Flux get concentrated on one pole but dispersed over the other and intervening
surfaces.
• So the asymmetry, in the alternator the inertia of rotation is very large and the
rotor to stator air gap is very small so less chance to permanent damage.
• At first on the field fault it must be detected and the set tripped in a controlled
manner, arrangement for earth fault detection and protection given in to figure.

Protection against rotor fault

Over-speeding
• Conceder turbo alternator supplying rated power at that time mechanical near
about it’s synchronous speed so at some reason some fault occurs at that time
generator will get tripped so it does not serve any electrical load but constantly
mechanical input is there so at that time abruptly speed go to very high value to
avoid it need to stop steam input should be zero
• For that the speed governing system of the turbine is responsible for detecting this
condition. The over speeding can also be detected either on the generator shaft.
Over speeding can be detected by over frequency relay also.

Over speed ?
Over frequency relay
Techogenerator
output
NO
YES
STOP STEM INPUT
START GENERATOR SHUT DOWN

Loss of excitation
• Possible cause to loss of excitation given below
• Loss of field to main exciter
• Accidental tripping of the field breaker
• Short circuit in the field winding
• Poor brush contact in the breaker
• Loss of ac supply to excitation system

Continue…
• The generator delivers both real and reactive power to grid, real power comes
from the turbine while the reactive power is due to the field excitation.
• Corresponding real power from rotor is Pm and corresponding reactive power
from field Qe,

Continue…

Loss of prime mover

Connection of generator
• Direct connection: A generator which connect directly to the load is called direct
connection.
• Via generating transformer: A generator which is connect indirectly with
transformer which step up the voltage.

Protection of prime mover failure

Continue…Phasor diagram

How to add property of direction in the relay

Torque develop strategy
`
𝜃
𝜙
𝜙 𝑣
𝜙𝐼
= 𝜙 𝑉 ∝ 𝑉
= 𝜙𝐼 ∝ 𝐼
𝜃 + 𝜙 = 𝜏
𝜙 + 𝜏 = 90
𝜙 = 90 − 𝜏
ሻ= KVIsin(𝜃 + 90 − 𝜏
ሻ= KVIsin(𝜃 + 𝜙
ሻ= KVIcos(𝜃 − 𝜏
So, will increase then speed of operation will increases

Generator protection requirements
• To detect faults on the generator
• To protect generator from the effects of abnormal power system operating
conditions
• To isolate generator from system faults not cleared remotely
• Action required depends upon the nature of the fault.
• Usual to segregate protection functions into :
−Tripping
−Alarm

Earth fault protection
• If there is no protection then what happen?
• An earth fault involving the stator core results in burning of the iron at the point of
fault and welds laminations together
• Earth fault protection is therefore a principle feature of any generator protection
Type of
protection
Method of
protection
Connection
of protectionNIRMA LTD. BHAVNAGAR

Method of earthing
• Mainly three types of earthing
• Neutral grounding resistance earthing
1. lossy method
2. not useful for higher rating of generators
• Neutral grounding transformer earthing
1. lossless earthing method
2. size is more because of transformer
3. Use for higher rating of generators
• Solidly earthing
1. Rapid damage occurs
2. burning the core iron
3. welding the core laminations

Continue… method of earthing

Why ground?
• Improve safety by allowing detection of faulted equipment
• Stop transient overvoltage
• Ability to detect a ground fault before a multiphase to ground fault evolves
• If impedance is introduced, limit ground fault current and associated damage
faults
• Provide good source of protection for other system.

Continue…
• The lower the R the better the ground source
• The lower the R more damage to the generator on
internal ground fault
• Can expansive resistor as rating goes up.
• Ground current typically 200 to 400A.

High impedance
• System ground source obtain from GSU.
• Uses principle reflects:
• Ground fault current typically: <=10A.
𝑅 𝑁𝐺𝑅 =
𝑅 𝑅
𝑉𝑃𝑅𝐼
𝑉𝑆𝐸𝐶

Compensated.
• Most expensive because tuned reactor and
transformer.
• Uses reflected impedance from grounding
transformer, same as high impedance ground system
does
• Generator fault mitigated from ground fault.
• Reactor tuned against generator capacitance to
ground to limit ground fault current to very low
value. (can be less than 1A)

Hybrid impedance grounding
• Has advantages of Low Z and High Z.
• Low Z grounded machine provides ground source for
other under normal condition.
• 51G acts as back protection for uncleared system
ground fault.
• 51G is too slow to protect generator for internal fault.
• Ground fault detected by 87GD element.
• Low Z path opened by vacuum switch.
• Only HighZ ground path is then available ao the high
ground path is limits the curret approximately 10A.

Only 10A for 60 cycles

Stator ground fault
• Traditional stator ground fault protection schemes include:
• Neutral over voltage
• Various third harmonic voltage dependent schemes.
• These exhibits sensitivity, security and clearing speed issue that may
subject a generator to prolonged low level ground faults that evolve
into damaging faults.

Stator ground fault
59G element covers 90-95% stator winding coverage

NGT ratio selected that provides 120 to 240V for ground
fault at machine terminals
Max. L-G volts: 11/1.73 = 6.35kv.
Max. NGT volts at sec.= 6350/120=52.91VTRNIRMA LTD. BHAVNAGAR

Why do we care about faults near neutral?
• A fault at or near the neutral shunts the high resistance that saves the
stator from large currents with an internal ground fault.
• If generator operating with an undetected with ground fault near to
neutral is a accident waiting to happen.
• We can use third harmonic or injection technique for complete
coverage.

Pole slipping protection
• The variation of impedance seen at the generator during pole
slipping.
• Trip signal is sent if the measured impedance crosses two
characteristics can be applied to increase the protection selectivity,
such as one or two blinders, MHO or a lenticular characteristics.
• The θ and blinder depends on the value of impedance

Pole slipping protection

• The impedance of the system and generator transformer determines the
forward reach of the lens, ZA and the transient reactance reach ZB.
• Width of the lens is set by the angle α and the line pp’, perpendicular to the
axis of the lens, is used to determine if the center of the impedance swing
during a transient is located in the generator.
• Normal operation is with the measured impedance in zone R1 if a pole slip
develops, the impedance locus will traverse through zone R2,R3, R4, when
entering R4 zone trip signal is issued provided impedance lies below
reactance line pp’ and hence the locus of swing lies within or close to the
generator, the generator is pole slipping with respect to the rest of the
system.

• If the locus lies above line PP’, the swing lies far out in the power
system- i.e. one part of the power system, including the protected
generator, is swinging against the rest of the network. Tripping may
still occur, but only if swinging is prolonged meaning that the power
system is in danger of complete break up. Further confidence checks
are introduce by requiring that the impedance locus spends a
minimum time within each zone for the pole slipping condition to be
valid.

• The trip signal may also be delayed for a number of slip cycles even if
a generator pole slip occurs this is to both provided confirmation of a
pole slipping condition and allow time for other relays to operate if
the cause of the pole slip lies somewhere in the power system.
Should the impedance locus traverse the zones in any other
sequence, tripping is blocked.

Generator Excitation System
• Main task of the excitation is to provide an adjustable DC current to
excite the winding of the generator, to meet the requirement for
generator normal operation. Controlling the excitation of the
synchronous generator is one of the main means to control generator
operation.
• Function of the excitation system:
1. Maintain the end voltage of the generator
2. Control the disturbution of reactive power
3. Improve the stability of synchronous generator paralleling
operation.

• Alternator mainly two types are following:
1. Separately excited system
2. self excited system.
• Separately excited system have PMG powering the automatic voltage
regulator and there is one isolation transformer for sensing the
voltage for AVR and in the other hand for self-excited alternator, there
is residual magnetism in the exciter field to initialize the voltage ramp-
up process and after voltage build-up, isolation transformer which is
responsible for voltage sensing for AVR, start powering the AVR as
well.

Continue…
• Self excited generator is responsible for two things.
1. Sensing voltage
2. Powering AVR after initial voltage build-up due to residual
magnetism.

Separately excited generator- PMG

Self excited alternator problem- lack of
residual magnetism
• Some times there is no voltage build up in the self excited alternator,
the reason is lack of residual magnetism.
• So for that remove the AVR need to connect the field with the battery
and blocking diode to the exciter field and after that start the engine,
if you see now voltage is building up, then It is confirm that exciter
field has been flashed successfully. Now stop the engine and remove
the battery and blocking diode , connect the AVR and again start the
engine to make sure that the alternator is back in operation.

Advantage of PMG Excitation support systems
• A PMG is appropriate regulator which provide constant voltage input
to the generator’s terminal voltage.
• When the load on the generator is nonlinear due to thyristor power
supplies such as UPS system.
• The load may produce notches on the power rectifiers in a shunt
excited generator’s AVR. When this occur generator’s terminal voltage
get unstable.
• PMG have very strong magnetic field. It eliminates field flashing.
• Strong and very fast voltage build-up.

THANK YOU
ANY QUESTION???

GENERATOR PROTECTIONS

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